The following description of the background of the invention is provided to aid in understanding the invention, but is not admitted to be, or to describe, prior art to the invention. All publications are incorporated by reference in their entirety.
Chlorinated hydrocarbons have been widely used for several decades. Improper handling and storage along with the widespread usage, has led to extensive soil and groundwater contamination. These materials are among the most prevalent groundwater contaminants in the United States today. Contamination of groundwater by chlorinated hydrocarbons is an environmental concern because these compounds have known toxic and carcinogenic effects. Chlorinated hydrocarbons (CHCs) pose both environmental and health risks. CHCs can be highly toxic, and some have been found to be potential mutagens and carcinogens. This group of chemicals, which include DDT and DDE, were banned starting in the 70's with more of the volatile members of the group such as TCE being banned recently. They have been found to be very resistant to decay and to biodegradation.
Pump and treat methodologies historically and presently have been used to remove contaminants from water and the subsurface. This technique uses a series of extraction wells drilled into a contaminated aquifer, and the contaminated water is drawn through the extraction well and treated to remove or to degrade the contaminants. The water is then returned to the aquifer via an injection well. This technology typically is very expensive and is used in conjunction with other methodology such as, air sparging and soil vapor extraction. A Federal Remediation Round Table report issued in 2004 indicated that it took 13 years to remove a total of 958 pounds of volatile organic compounds (VOCs) at a site in Gresham, Oreg. at a cost of $2,540 per pound of VOC. (URL:http://costperformance.org/profile.cfm? ID=370 & CaseID=370 Page last modified on: Thursday, Apr. 7, 2005)
Anaerobic bacteria have also been employed in attempts to biodegrade chlorinated hydrocarbons in situ. Some species perform this function through reductive dechlorination. This process requires a steady supply of an electron donor such as hydrogen. Several methods have been proposed to supply this hydrogen (U.S. Pat. No. 5,277,815 and U.S. Pat. No. 5,602,296). As the hydrogen is immediately released in the treatment area, there is a need to constantly inject large volumes of solutions into the systems. The use of polymers in the form of granules, briquettes, pellets, tablets, and capsules have been attempted to provide slow release of soluble and insoluble organic substrates as a means of enhancing anaerobic bioremediation (Hince U.S. Pat. No. 6,620,611 B2). Fuel cells including multi-metallic particles have been used in a similar fashion (Hitchens et al. U.S. Pat. No. 6,265,205 B2). Zero valent iron has also been proposed to treat groundwater and surface streams for decontamination (Quinn et. al, U.S. Pat. No. 7,008,964 and Ponder et al, U.S. Pat. No. 6,242,663).
Farone et al. (U.S. Pat. No. 6,420,594) disclosed the composition and use of a series of polylactate esters that when placed in water under selected conditions slowly hydrolyze to release lactic acid. As the lactic acid is assimilated, it is gradually converted to acetic acid releasing hydrogen which is utilized by microbes to assimilate various chemicals such as chlorinated solvents, pesticides, and explosives that are present.
There have been attempts to use vegetable oils for a similar purpose (U.S. Pat. No. 5,265,674). Vegetable oils are not as biochemically efficient at producing hydrogen and they have the effect of being slower to be utilized by methanogenic bacteria thus keeping the oxidation reduction potential (ORP) in the ground water above the point of producing significant amounts of methane. A disadvantage of vegetable oil is that it is mobile and will move in the ground water. It is difficult to use vegetable oil as an injectable barrier such as that which the polylactate esters provide. In addition the movement of the oil can also mobilize the pools of contaminants making it difficult to ascertain where the oil/contaminants are traveling in the aquifer. The most rapid rate of decomposition of most of these contaminants occurs under acetogenic conditions (ORP is −100 mv to −200 mv). Under methanogenic conditions most of the chemical energy goes into producing methane and is therefore wasted (ORP is below −200 mv). Maintaining the correct ORP in the acetogenic range allows the degradation of the contaminants to proceed at a more rapid rate. This ORP effect is due to the fatty acids released from the oil by hydrolysis or partial utilization of the oil and not from the oil itself.
All of the previously mentioned methods have been shown to have serious shortcomings. Addition of non native materials adds additional foreign material to an already contaminated site. With materials that release immediately there is a requirement for frequent addition of remediation matter. This frequent addition of the chosen material is needed to keep a sufficient concentration in the contaminated area over time. The constant injection of high volumes of solutions will increase the volume of the system or aquifer and thereby potentially cause further spread of the contamination. Special measures are required to deoxygenate the water and solutions which are injected, to ensure maintaining the anaerobic atmosphere which fosters the reduction. Thus a need exists for a safe, cost-effective method for removing these contaminating materials from ground water and soils.